The specific goal of this project is to study in depth the role of immunity in the natural history of Mycoplasma pneumoniae infection. The immunity status and the frequency of reinfection in persons who have had M. pneumoniae pneumonia in the last eight years will be sought, and the role of different types of antibody in protection from illness or infection will be analyzed. The possible role of hypersensitivity causing enhanced disease response will be evaluated. The Group Health Cooperative in Seattle, provides an ideal setting for longitudinal studies of this infection in a stable population group. Through eight years of studies, 800 individuals have been identified as having had M. Pneumoniae pneumonia as proven by isolation. Sera collected from these individuals at intervals after infection as well as sera from persons in families and schools obtained at the time or prior to their exposure to natural challenge, furnish ideal material for evaluating protective antibody. Strain variation in reinfection and between the two epidemics will be sought.